The invention described herein relates to an improved method for the consistent production of rhombic and barrel shaped precipitated calcium carbonate (PCC). The process allows enhanced control of the surface area of PCC, higher starting carbonation temperatures, more tolerance to the fluctuations in the starting carbonation temperatures, high solids concentration, higher starting temperature for slaking, and obviates the need to control the temperature during slaking and carbonation.
Calcium carbonate pigments have found increasing application as fillers, and also as coating pigments, in the manufacture of paper and paperboard. This utility arises by virtue of their excellent optical and physical properties when used for such purposes.
The consistent production of rhombic and barrel shaped PCC from various limes has not been achieved previously. The production of rhombic and barrel shaped PCC utilizing temperature control alone has seemed to provide a "hit or miss" approach in the lab as well as in commercial operations. Inconsistent results are typical with a tendency to produce very high surface area PCC--for example having BET surface areas greater than 20 m.sup.2 /g). Because of this tendency to produce very high surface area PCC, the control of starting carbonation temperatures has become critical, since small temperature variations can produce large surface area fluctuations.
To avoid these prior inconsistencies, alternatives such as lower starting slaking and carbonation temperatures and lower solids concentration have been attempted with mixed results. It also has been disclosed in the art to reduce the sensitivity of the PCC production processes by the use of certain additives. For example, U.S. Pat. No. 2,188,663 (McClure et al,) relates to a process of making fine particle alkaline earth metal carbonates by reacting an aqueous suspension of Ca(OH).sub.2 with carbon dioxide to form substantially colloidal CaCO.sub.3. By addition of certain agents to the Ca(OH).sub.2 suspension, including certain saccharides, the temperature range at which the reaction can be conducted is said to be widened to avoid the need for rigid temperature control.
Additional prior art patents of pertinence to this invention include the following:
U.S. Pat. No. 2,467,082 (Fleck) relates to the manufacture of CaCO.sub.3, and the use of a sugar-free extract from sugar beet residue for inhibiting the growth of CaCO.sub.3 crystals precipitated from a Ca(OH).sub.2 slurry by carbonation.
U.S. Pat. No. 3,320,026 (Waldeck) teaches the production of rhombic PCC by employing lower hydroxide concentrations, lower slaking temperature, and controlling the maximum slaking and carbonation temperatures to a lower limit.
U.S. Pat. No. 3,433,890 (Sisson et al.) relates to the production of precipitated CaCO.sub.3 of purportedly uniform and fine ultimate particle size. Particles less than about 0.1 micron are precipitated from an aqueous suspension of Ca(OH).sub.2 in the presence of about 0.2 to 5% by weight of a saccharide or polyol and about 0.2 to 5% of an active CO.sub.2 compound.
U.S. Pat. No. 3,669,620 (Bennett et al.) relates to needle-shaped (aragonite) crystals of CaCO.sub.3 made by dissolving calcium hydroxide in an aqueous sucrose solution, and then treating the solution with carbon dioxide. The aragonite needle-shaped crystals can be produced in lengths up to about 50 microns.
None of the above patents addresses the consistent production of PCC having rhombic or barrel shaped crystals and a BET surface area less than about 20 m.sup.2 /g.
The terms "rhombic" and "barrel-shaped" are used qualitatively when appropriate herein to refer to the overall shape of the PCC particles. The term "rhombic" indicates an aspect ratio of 1.5 or less, whereas "barrel" indicates an aspect ratio higher than 1.5.
The term "aspect ratio" as used herein is defined as the ratio of the length of the longest side of the crystal to the diameter of the shortest side of the crystal. The aspect ratio can be a number, e.g. 1 when all the crystals are uniform with an L/D ratio of 1, or it can be a range, for example 1 to 1.5 when a mixture of crystals is present having as aspect ratio values from 1 to 1.5.
The present invention eliminates the aforementioned problems to provide rhombic or barrel-shaped PCC of extremely uniform particle size and surface area; it allows use of higher solids concentration, higher slaking temperatures, and eliminates the need to control maximum slaking and carbonation temperatures.